Testosterone stimulates both cell enlargement and cell division in muscle. This project is designed to evaluate the hypothesis that the hormone-induced hypertrophy and hyperplasia occur by biochemical distinguishable mechanisms, and can occur essentially independently of each other. The study will be done on muscle cells in tissue culture; primary cultures or established muscle cell lines (L6 and M3A) will be utilized where their properties are advantageous. The project will be divided into three parts: (1) Development of rapid and convenient assays for hyperplastic and hypertrophic effects, (2) Comparison of activities of various testosterone metabolites, analogs, and antagonists as stimulators of cell enlargement and cell division, and (3) Characterization of the interaction between testosterone (or its metabolites) and muscle cells. This experimental approach should demonstrate differences in hyperplastic and hypertrophic effects of testosterone if they occur at the level of the effector, the receptor, or the mechanisms of action; we anticipate that these determinations will provide important indications of the mechanisms by which the myogenic action of testosterone is expressed. The testosterone-muscle system is of particular interest because testosterone is the basis for many drugs designed as protein anabolic agents, and because it now appears that the action of testosterone on muscle occurs by mechanisms different from ay other steroid hormone- target tissue interaction. Thus the proposed study has important practical as well as theoretical significance.